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Dec 4, 2017 - B. Li, Dr. P. Gu, G. X. Zhang, Y. Lu, K. S. Huang, Prof. H. G. Xue,. Prof. H. Pang. School of Chemistry and Chemical Engineering. Institute for ...
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Ultrathin Nanosheet Assembled Sn0.91Co0.19S2 Nanocages with Exposed (100) Facets for High-Performance Lithium-Ion Batteries Bing Li, Peng Gu, Guangxun Zhang, Yao Lu, Kesheng Huang, Huaiguo Xue, and Huan Pang* shows significant potential for nanoelectronic applications owing to its high carrier mobility, eco-friendliness, low cost, structural stability, and excellent electrochemical performance.[15–17] SnS2 and its derivatives have been extensively investigated for photocatalysis, dye-sensitized solar cells, and lithium-ion batteries.[18,19] The mechanism to store lithium for SnS2 is on the basis of the Li+ alloying/ dealloying process, where the formation/deformation of Li4.4Sn alloy is the intrinsic driving force.[20] However, the main problem is the volume expansion, slow ionic transportation, and the mecha­ nical stress by the accumulation of large amounts of lithium, which may lead to a rapid deterioration and a low-capacitance retention.[20,21] These chemical and physical changes mentioned could also result in the loss of electrical connectivity between the electroactive materials and current collector. The strategies for solving above problems are closely related to the compensation of the volume change, the improvement of the ionic transportation, and the enhancement of the system’s electronic conductivity.[22] Morphology control is an effective way to shorten the pathway of lithium ions and compensate the volume change by designing hollow, porous, or other unique nanostructures.[23] Another strategy is to improve the conductivity of Sn-based materials through adding electronically conductive agents,[24,25] which has been demonstrated as a cost-effective strategy to manipulate the physical and chemical properties of metal oxides and sulfides, especially for the altering of crystal structures by introducing defects.[26] For Li-ion batteries, by tuning the materials’ surface morphologies, different facets could be exposed and cause various atomic arrangements on the surface and may lead to the modified absorption and desorption of Li ions.[27–30] For instance, Xiao et al. synthesized three types of Co3O4 with independent well-defined crystal plane structures. The electrochemical results show that the Co3O4 with the (111) plane exposure displayed the highest reversible capacity and the best rate capability.[30] In this work, we designed and synthesized ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages to reduce the volume expansion, improve the ionic transportation, and enhance the overall electronic conductivity. We prepared CoSn(OH)6 nanocubes as precursor, Thioacetamide (TAA)

Ultrathin 2D inorganic nanomaterials are good candidates for lithium-ion batteries, as well as the micro/nanocage structures with unique and tunable morphologies. Meanwhile, as a cost-effective method, chemical doping plays a vital role in manipulating physical and chemical properties of metal oxides and sulfides. Thus, the design of ultrathin, hollow, and chemical doped metal sulfides shows great promise for the application of Li-ion batteries by shortening the diffusion pathway of Li ions as well as minimizing the electrode volume change. Herein, ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages with exposed (100) facets are first synthesized. The as-prepared electrode delivers an excellent discharge capacity of 809 mA h g−1 at a current density of 100 mA g−1 with a 91% retention after 60 discharge–charge cycles. The electrochemical performance reveals that the Li-ion batteries prepared by Sn0.91Co0.19S2 nanocages have high capacity and great cycling stability. Ultrathin 2D nanomaterials have attracted remarkable attention on Li-ion batteries,[1,2] as well as the nanocages metal oxides/ sulfides with extraordinary structure enabled features.[3–8] Therefore, the combination of nanocages and 2D nanosheets should present some benefits in manipulating materials’ physical and chemical properties. First, the nanocages could offer larger specific surface area and lead to a better surface contact area between the electrolyte and electrode to enhance diffusion of lithium ions and electrons which could address the challenges of capacity retention and rate capacity.[9] Second, the ultrathin nanosheet structure may shorten the ion/electron transport path and improve the power density and rate capability.[10–12] Third, the ultrathin nanosheet assembled nanocage architecture could sustain their structure and the exterior sheets prevent the internal dissolution, giving rise to a better cycling performance.[13,14] As Sn2+ is inherently unstable and easy to be oxidized to Sn4+, thin layer of tin dichalcogenide (SnS2), instead of SnS, B. Li, Dr. P. Gu, G. X. Zhang, Y. Lu, K. S. Huang, Prof. H. G. Xue, Prof. H. Pang School of Chemistry and Chemical Engineering Institute for Innovative Materials and Energy Yangzhou University Yangzhou, 225002 Jiangsu, P. R. China E-mail: [email protected], [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201702184.

DOI: 10.1002/smll.201702184

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Figure 1.  The synthesis schematic diagram of the ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages.

as the sulfur source and ethylenediaminetetraacetic acid (H4EDTA) as the complexing agent. By tuning the factors of temperature and time, we found that the sample at 180 °C with 4 h reaction time could show ultrathin nanosheet assembled nanocages (Sn0.91Co0.19S2) with the (100) plane exposure. Furthermore, we used SnCl4 instead of CoSn(OH)6 nanocubes as the reaction reagent, while keeping other conditions unchanged obtained pure SnS2 microflowers. Importantly, the sample of ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages with the (100) planes exposure showed a better lithium-ion storage capacity and cycling stability. Initially, the CoSn(OH)6 nanocubes were synthesized, and the scanning electron microscopy (SEM) images and energydispersive spectrometer mapping images of CoSn(OH)6

nanocubes are shown in Figure S1 (Supporting Information). The synthesis of Sn0.91Co0.19S2 can be performed with the asprepared CoSn(OH)6 nanocubes as the precursor, TAA as the sulfur source, and H4EDTA as the complexing agent. Through a hydrothermal process, H4EDTA can extract Co2+ ions and hold them tightly forming a soluble complex Co-H2EDTA with a complexation stability constant of lg Kfθ = 16.31. Meanwhile, Sn4+ ions were slowly released and then reacted with S2− from TAA decomposition. The reaction process is shown in Figure 1. The crystallographic structure and phase purity of the asprepared samples were examined by X-ray powder diffraction (XRD) measurement. In Figure 2a, the diffraction peaks are mainly assigned to SnS2 (JCPDS No. 83-1705) with space group of P-3m1. In the XRD pattern, major diffraction peaks at 15.1°,

Figure 2.  a) XRD patterns of the ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages and pure SnS2, and XPS spectra of the ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages: b) Co 2p, c) Sn 3d, d) S 2p.

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28.3°, 32.2°, 42.0°, 50.1°, 52.6°, and 60.8° can be indexed to the (001), (100), (011), (012), (110), (111), and (201) facets of the SnS2 phase. The intensity of the peak at 28.3° indexed as (100) is significantly enhanced in comparison with that of the control sample, indicating that the sample mostly exposes the facets of the (100) plane. To further explore the element composition and valence states of the as-prepared materials, an attempt was made to access the binding energy and chemical states by X-ray photoelectron spectroscopy (XPS) measurement. The corresponding high-resolution spectra is shown in Figure 2b–d. Figure 2b shows the high-resolution spectrum of Sn0.91Co0.19S2 that can be best fitted by Co 2p1/2 and Co 2p3/2 peaks located at 794.6 and 779.7 eV, which distinctly verifies the presence of Co(II). Figure 2c displays the high-resolution XPS spectrum of Sn 3d that can be best fitted by Sn 3d3/2 and Sn 3d5/2 peaks located at 495.1 and 486.7 eV, which distinctly verifies the presence of Sn(VI). As shown in Figure 2d, the high-resolution S 2p XPS spectrum can be deconvoluted into two spin–orbit doublets and two shake-up satellites, in which the binding energy for S 2p1/2 and S 2p3/2 is observed at 162.9 and 161.7 eV, respectively, indicating the existence of the S(II) state. In addition, this material contains elements of Co, Sn, and S with an atomic ratio of 0.21:1.0:2.0 shown by XPS. Combined with the results from XRD, the material probably composed of tin sulfide and cobalt sulfide, and the product can be named Sn0.91Co0.19S2 according to the atomic percentage. Figure S2 (Supporting Information) shows the crystal structure of the SnS2 super cells based on data from the inorganic crystal structure database (ICSD-43004). It is clear that the sample shows layered structure, and the distance of neighboring layer is 0.59 nm. As shown in Figure S2 (Supporting Information), Li+ can easily transport between adjacent layers, so the (100) and (010) planes are good for the Li+ transportation. However, it is difficult for Li+ to transport through the (001) plane. In combination with the XRD, the sample exposed more (100) planes while hiding most of the (001) planes, which may be favorable for ion transportation and improve its electrochemical performance. The morphology of as-prepared sample was also tested by transmission electron microscopy (TEM). A typical low-magnification TEM image in Figure 3a exhibits that the morphology of the as-obtained sample is ultrathin nanosheet assembled cage structure. Figure 3b shows the scanning transmission electron microscopy image of the sample, and it is more intuitive to testify that the morphology is ultrathin nanosheet assembled nanocage. Cage structure is more conducive to ion transmission, and the ultrathin nanosheet structure can provide sufficient electroactive sites.[26,31] Figure 3c shows the high-resolution TEM (HRTEM) image of the nanosheets. The HRTEM image exhibits the atomic lattice fringe spacing of 0.315 nm corresponding to the (100) plane of the Sn0.91Co0.19S2. As indicated in Figure 3d, the mapping of the elements Co, Sn, and S from a selected area suggests that those elements are homogeneously distributed in the whole area without obvious element aggregations or separations, which provides further evidence of the material Sn0.91Co0.19S2. As we all known, surface area and pore-size distribution play a vital role in mechanical, thermal, and chemical properties

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of the electroactive materials. Herein, the performance of as-prepared Sn0.91Co0.19S2 was further ascertained by the specific surface area and porosity via nitrogen sorption measurements. Figure S3a (Supporting Information) displays the N2 adsorption–desorption isotherm and corresponding pore-size distribution curve (inset) for the sample. The N2 adsorption– desorption isotherm displays an IV isotherm, demonstrating the existence of mesopores and macropores. The pore-size distribution curve has been obtained from desorption data and worked out from the isotherm by the Barrett–Joyner–Halenda (BJH) model. Some of these holes are caused by the cage structure, and others are formed by the disorder arrangement of the nanosheets. The specific surface area of the as-prepared sample received from N2 desorption is 79.0 m2 g−1 by Brunauer– Emmett–Teller (BET) measurement. The uniform CoSn(OH)6 nanocubes play an important role in the hydrothermal process. When SnCl4 is utilized as the reaction reagent rather than CoSn(OH)6 nanocubes, other conditions remained unchanged, the product is assigned to pure SnS2 (JCPDS No. 83-1705) according to the XRD pattern in Figure 2a. The pure SnS2 sample was denoted as A0. Figure S4 (Supporting Information) shows SEM and TEM images of sample A0 at different magnifications, and the morphology is microflower assembled with nanosheets. Figure S3b (Supporting Information) displays the N2 adsorption–desorption isotherm and corresponding pore-size distribution curve (inset) for the pure SnS2, and the BET-specific surface area calculated from N2 desorption is 11.5 m2 g−1. According to the above results, the larger surface area and cage structure may extremely improve the electrochemical performances of Sn0.91Co0.19S2. For the sake of exploration of the crystal growth process of Sn0.91Co0.19S2, the reaction time was efficiently controlled and changed. The samples with reaction times of 2, 4, 8, and 12 h at 180 °C were denoted as A1–A4. A sequence of reactions was conducted with increasing temperature, and the samples obtained under 200 °C for 2, 4, 8, and 12 h were denoted as A5–A8. Supporting Information and Figures S5–S9 therein show detailed change process of the materials’ morphology and composition. The reaction mechanism between SnS2 and Li is a conversion followed by an alloying/dealloying reaction, which is described in Equations (1) and (2). Due to irreversible decomposition of SnS2 and the formation of a solid electrolyte interface film in the first cycle, the initial coulombic efficiency of SnS2-based materials is commonly low.[20,31] The theoretical capacity is 645.5 mA h g−1, according to Li4.4Sn, and the first irreversible loss of SnS2 is 47.6% SnS2 + 4Li + + 4e − → Sn + 2Li 2S (1) Sn + x Li + + xe − ↔ LixSn (0 ≤ x ≤ 4.4) (2) The Li-ion storage performance of the as-prepared Sn0.91Co0.19S2 and pure SnS2 was studied by the galvanostatic charge–discharge test. Figure 4a shows the cycling stability of the as-prepared Sn0.91Co0.19S2 and pure SnS2 discharged at 100 mA g−1 for 60 cycles. Based on the mass of active material,

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improve the electronic conductivity and electrochemical performance. The nanosheets were getting larger and thicker as the reaction time was continually prolonged to 8 and 12 h, and in the end, microflower was formed, which was detrimental to electron transmission and prone to volume expansion. The discharge capacity of samples A3 and A4 is less than that of sample A2, and the capacity decays faster with the increase of cycle number. In order to investigate the electrochemical details of the Sn0.91Co0.19S2, cyclic voltammetric experiments were initially conducted between 0.01 and 3.0 V at a scan rate of 0.05 mV s−1. Cyclic voltammograms of the Sn0.91Co0.19S2 electrodes for lithium-ion cells are shown in Figure 4b. Broad peaks are present at about 0.58, 1.15, 1.45, and 2.27 V in the first potential sweeping process, where the peak at ≈2.27 V is not yet observed in the later cycles. This could be attributed to the intercalation of lithium ions into the SnS2 layers without phase decomposition.[32] The peaks of 1.15 and 1.45 V could be ascribed to the decomposition of SnS2 into metallic Sn and Li2S, while the peak at around 0.58 V is related to the subsequent generation of the Li–Sn alloy. By comparing the second cycle with the fifth cycle, it is obvious that the Figure 3.  a,b) TEM images and c) high-resolution TEM image of the ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages at different magnifications. d) Elemental mapping showing the curve of the Sn0.91Co0.19S2 is quite similar, homogeneous distribution of all elements (Co, Sn, and S). which indicates a relatively stable charge–discharge process. Galvanostatic charge–discharge curves of sample A2, the initial discharge capacity of samples A0–A4 is 1908.3, 421.6, A0, A1, A3, and A4 electrodes are shown in Figure 4c and 1836.3, 1935.2, and 1525.7 mA h g−1, respectively, with the first Figure S10a–d (Supporting Information), which were cycled irreversible loss of 47.7%, 50.5%, 46.0%, 47.5%, and 60.7%. at the 1st, 10th, 20th, 30th, 40th, 50th, and 60th between 0.01 Although the capacity of pure SnS2 in the first few cycles is and 1.5 V (current density of 200 mA g−1). These figures furhigher than that of the Sn0.91Co0.19S2 for 4 h, it decays rapidly after several cycles. The initial coulombic efficiency and electrother prove that sample A2 displays the best lithium-ion storage chemical stability of the Sn0.91Co0.19S2 are higher than those of capacity and cycling stability. It is clear that the first discharge and charge capacity of sample A2 is about 1436 mA h g−1, with the pure SnS2, which may be due to the special morphology of Sn0.91Co0.19S2. As indicated by SEM images, these Sn0.91Co0.19S2 coulombic efficiency about 51.2%, and the discharge capacity can retain 637.9 mA h g−1 after 60 cycles. maintain the cube framework, effectively reducing the adverse [9,14] effects of volume expansion. To understand the electrode kinetics, the as-prepared A0–A4 for lithium intercalation were estimated by electrochemical Sample A1 delivers a low capacitance, which is probably impedance spectra in the frequency from 0.01 to 105 Hz under caused by the not completed reactions in 2 h time and many complexes contained. When the reaction time was prolonged to open-circuit voltages, which are shown in Figure S11 (Sup4 h, Sn4+ ions were fully released and the reaction was almost porting Information). In the high-frequency region, the smaller semicircle diameter for the A2 electrode indicates a smaller completed. At a current density of 100 mA g−1, the sample charge transfer impedance compared to the others electrode, A2 had an average capacity of about 809 mA h g−1 and mainsuggesting quicker charge transfer. In the lower frequency tained 730 mA h g−1 after 60 cycles, which exhibits the highest area, the slope of the curve shows the Warburg impedance that capacity and excellent cycling stability. The reason for the high corresponds to the diffusion of lithium ions. A2 electrode is capacity and good cycling stability of the sample may be narthe most ideal straight line among them, which demonstrates rated as follows. (1) The sample maintains the cube frame and that it has the lowest diffusion resistance. The lower resistthe ultrathin nanosheet assembled nanocages, which can effecance of A2 electrode also indicates that the conductive ultrathin tively reduce the adverse effects of volume expansion. (2) The nanosheet assembled nanocages provide a rapid route for higher percentage of (100) facet exposure may help Li+ absorplithium-ion transport and concurrently shorten the diffusion tion and desorption, and eventually modify the electrochemical distance. performance. (3) Doped cobalt ions in SnS2 may significantly

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Figure 4.  a) Samples A0–A4 discharged at 100 mA g−1 for 60 cycles. b) Cyclic voltammogram of sample A2 at a scan rate of 0.05 mV s−1. c) Galvanostatic charge–discharge curves of sample A2, cycled at the 1st, 10th, 20th, 30th, 40th, 50th, and 60th between 1 mV and 1. 5 V versus Li/Li+ at a current density of 200 mA g−1. d) Rate capability of the sample A2 electrode at various current rates between 100 mA g−1 and 10 A g−1.

To further test the cyclability and rate capability of sample A2, rate capability was measured over cycling at various current densities from 100 mA g−1 to 10 A g−1, and then back to 100 mA g−1 for 20 cycles. As shown in Figure 4d, the discharge capacity is 809.9 mA h g−1 after 20 cycles at 100 mA g−1. With the increase in the current density, the average capacities are 766.8, 727.2, 666.1, 593.6 and 487.1 mA h g−1 at 200 mA g−1, 500 mA g−1, 1 A g−1, 2 A g−1, and 10 A g−1, respectively. When the current density is set back to 1 A g−1 after 120 cycles, the capacity recovers to 637.5 mA h g−1, indicating a marvelous capacity retention of 95.6%. Figure S11a–d (Supporting Information) shows that the cycling stability of sample A2 discharged at 100 mA g−1, 200 mA g−1, 500 mA g−1 and 1 A g−1 for 60 cycles, respectively, which reveal that the capacity can maintain stability at different current densities. A comparison of previously reported SnS2 and its derivatives is given in Table S1,[32–40] from which we can see that the discharge capacity or stability of pure SnS2 is not as good as that of SnS2 combined with other material. The morphology is of vital importance for the capacitance performance and cycling stability. The reported 2D SnS2-based materials exhibit excellent cycling stability; however, the capacitance performance still needs to be improved.[1,32] In our work, the Sn0.91Co0.19S2 not only shows high discharge capacity, but also delivers good cycling stability, which can be attributed to the special structure, the metal doping, and the large percentage of exposed (100) planes. The ultrathin nanosheet assembled nanocages can offer larger effective specific surface area, short transport length, and improve electrical activity for both ions and electrons, and effectively reduce the adverse effects of volume expansion. Metal doping significantly improves electronic conductivity and electrochemical properties. Exposure of (100) planes and the hiding of (001) planes facilitate the transmission of ions.

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We have successfully synthesized the ultrathin nanosheet assembled Sn0.91Co0.19S2 nanocages via a hydrothermal reaction, prepared by CoSn(OH)6 nanocube precursor, complexation of cobalt ions and H4EDTA, and Sn4+ ions reacting with S2− derived from TAA decomposition subsequently. The influence factors of reaction temperature and reaction time were explored on the growth of the as-prepared materials. It is found that Sn0.91Co0.19S2 obtained at 180 °C with the reaction time of 4 h displays the best lithium-ion storage capacity and cycling stability. The good electrochemical performance of the Sn0.91Co0.19S2 can be attributed to the unique structure, chemical doping, and exposure of (100) planes. This simple and effective synthesis method can be widely applied to the preparation of other cage structure metal-doped SnS2 for highperformance Li-ion batteries.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the Program for New Century Excellent Talents of the University in China (Grant No. NCET-13-0645), the National Natural Science Foundation of China (Grant Nos. NSFC21201010, 21671170, and 21673203), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 164200510018), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (Grant No. 14IRTSTHN004), the Six Talent Plan (Grant No. 2015-XCL-030) the Qinglan Project, and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015B112). The authors

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Conflict of Interest The authors declare no conflict of interest.

Keywords lithium-ion batteries, nanocages, nanosheets, ultrathin Received: June 27, 2017 Revised: October 9, 2017 Published online:

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